Search | Glossary
CCNA Exploration - Accessing the WAN
2.0 Chapter Introduction
2.0.1 Chapter Introduction
This chapter starts your exploration of WAN technologies by introducing point-to-point communications and the Point-to-Point Protocol (PPP).
One of the most common types of WAN connection is the point-to-point connection. Point-to-point connections are used to connect LANs to service provider WANs, and to connect LAN segments within an Enterprise network. A LAN-to-WAN point-to-point connection is also referred to as a serial connection or leased-line connection, because the lines are leased from a carrier (usually a telephone company) and are dedicated for use by the company leasing the lines. Companies pay for a continuous connection between two remote sites, and the line is continuously active and available. Understanding how point-to-point communication links function to provide access to a WAN is important to an overall understanding of how WANs function.
Point-to-Point Protocol (PPP) provides multiprotocol LAN-to-WAN connections handling TCP/IP, Internetwork Packet Exchange (IPX), and AppleTalk simultaneously. It can be used
over twisted pair, fiber-optic lines, and satellite transmission. PPP provides transport over ATM, Frame Relay, ISDN and optical links. In modern networks, security is a key concern. PPP allows you to authenticate connections using either Password Authentication Protocol (PAP) or the more effective Challenge Handshake Authentication Protocol (CHAP). These
are taught in the fourth section.
In this chapter you will also learn the key concepts of serial communications, and how to configure and troubleshoot a PPP serial connection on a Cisco router.
2.0.1 - Chapter Introduction
The diagram depicts the chapter objectives:
- Describe the fundamental concepts of point-to-point serial communication.
- Describe key P P P concepts.
- Configure P P P encapsulation.
- Explain and configure PAP and CHAP authentication.
2.1 Serial Point-to-Point Links
2.1.1 Introducing Serial Communications
How Does Serial Communication Work?
You know that most PCs have both serial and parallel ports. You also know that electricity can only move at one speed. One way to get bits to move faster through a wire is to compress the data so that less bits are necessary and then require less time on the wire, or transmit the bits simultaneously. Computers make use of relatively short parallel connections between interior components, but use a serial bus to convert signals for most external communications.
Let's compare serial and parallel communications.
Click the Serial and Parallel button to view the animation.
; With a serial connection, information is sent across one wire, one data bit at a time. The 9-
pin serial connector on most PCs uses two loops of wire, one in each direction, for data
communication, plus additional wires to control the flow of information. In any given
direction, data is still flowing over a single wire.
; A parallel connection sends the bits over more wires simultaneously. In the case of the 25-
pin parallel port on your PC, there are eight data-carrying wires to carry 8 bits simultaneously.
Because there are eight wires to carry the data, the parallel link theoretically transfers data
eight times faster than a serial connection. So based on this theory, a parallel connection
sends a byte in the time a serial connection sends a bit.
This explanation brings up some questions. What is meant by theoretically faster? If parallel is faster than serial, is parallel more suitable for connecting to a WAN? In reality, it is often
the case that serial links can be clocked considerably faster than parallel links, and they achieve a higher data rate, because of two factors that affect parallel communications: clock skew and crosstalk interference.
Click the Clock Skew button in the figure.
In a parallel connection, it is wrong to assume that the 8 bits leaving the sender at the same time arrive at the receiver at the same time. Rather, some of the bits get there later than others. This is known as clock skew. Overcoming clock skew is not trivial. The receiving end must synchronize itself with the transmitter and then wait until all the bits have arrived. The process of reading, waiting, latching, waiting for clock signal, and transmitting the 8 bits adds time to the transmission. In parallel communications, a latch is a data storage system used to store information in sequential logic systems. The more wires you use and the farther the connection reaches, compounds the problem and adds delay. The need for clocking slows parallel transmission well below theoretical expectations.
This is not a factor with serial links, because most serial links do not need clocking. Serial connections require fewer wires and cables. They occupy less space and can be better isolated from interference from other wires and cables.
Click the Interference button in the figure.
Parallel wires are physically bundled in a parallel cable, and signals can imprint themselves on each other. The possibility of crosstalk across the wires requires more processing, especially at higher frequencies. The serial buses on computers, including routers, compensate for crosstalk before transmitting the bits. Since serial cables have fewer wires, there is less crosstalk, and network devices transmit serial communications at higher, more efficient frequencies.
In most cases, serial communications are considerably cheaper to implement. Serial communications use fewer wires, cheaper cables, and fewer connector pins.
2.1.1 - Introducing Serial Communications
The diagram depicts the serial communication process across a WAN link. The 9-pin RS-232 serial connector and pins are also described.
Serial Communication Process:
A PC on one side of the WAN cloud is sending data that is to be encapsulated to the router. The data
is encapsulated by the communications protocol used by the router. The router sends the
encapsulated frame across the serial WAN link (physical medium) as a string of zeros and ones. At the
receiving end, a router uses the same communications protocol to de-encapsulate the frame. The
unencapsulated data is then sent to another PC.
RS-232 serial connector:
The diagram shows a 9-pin D-type RS-232 serial connector with a table listing each, an abbreviation
of its signal, and a description of what the pin does.
Description: Data carrier detect.
Description: Receive data.
Description: Transmit data.
Description: Data terminal ready.
Description: Signal ground.
Description: Data set ready.
Description: Request to send.
Description: Clear to send.
Signal: R I.
Description: Ring indicator.
Serial Communication Standards
All long-haul communications and most computer networks use serial connections, because the cost of cable and synchronization difficulties make parallel connections impractical. The most significant advantage is simpler wiring. Also, serial cables can be longer than parallel cables, because there is much less interaction (crosstalk) among the conductors in the cable.
In this chapter, we will confine our consideration of serial communications to those connecting LANs to WANs.
The figure is a simple representation of a serial communication. Data is encapsulated by the communications protocol used by the sending router. The encapsulated frame is sent on a physical medium to the WAN. There are various ways to traverse the WAN, but the receiving router uses the same communications protocol to de-encapsulate the frame when it arrives.
There are many different serial communication standards, each one using a different signaling method. There are three key serial communication standards affecting LAN-to-WAN connections:
; RS-232 - Most serial ports on personal computers conform to the RS-232C or newer RS-422
and RS-423 standards. Both 9-pin and 25-pin connectors are used. A serial port is a general-
purpose interface that can be used for almost any type of device, including modems, mice,
and printers. Many network devices use RJ-45 connectors that also conform to the RS-232
standard. The figure shows an example of an RS-232 connector.
; V.35 - Typically used for modem-to-multiplexer communication, this ITU standard for high-
speed, synchronous data exchange combines the bandwidth of several telephone circuits. In
the U.S., V.35 is the interface standard used by most routers and DSUs that connect to T1
carriers. V.35 cables are high-speed serial assemblies designed to support higher data rates
and connectivity between DTEs and DCEs over digital lines. There is more on DTEs and DCEs
later in this section.
; HSSI - A High-Speed Serial Interface (HSSI) supports transmission rates up to 52 Mb/s.
Engineers use HSSI to connect routers on LANs with WANs over high-speed lines such as T3
lines. Engineers also use HSSI to provide high-speed connectivity between LANs, using Token
Ring or Ethernet. HSSI is a DTE/DCE interface developed by Cisco Systems and T3plus
Networking to address the need for high-speed communication over WAN links.
Click the RS-232 button in the figure.
As well as using different signaling methods, each of these standards uses different types of cables and connectors. Each standard plays a different role in a LAN-to-WAN topology. While this course does not examine the details of V.35 and HSSI pinning schemes, a quick look at a 9-pin RS-232 connector used to connect a PC to a modem helps illustrate the concept. A later topic looks at V.35 and HSSI cables.
; Pin 1 - Data Carrier Detect (DCD) indicates that the carrier for the transmit data is ON.
; Pin 2 - The receive pin (RXD) carries data from the serial device to the computer.
; Pin 3 - The transmit pin (TxD) carries data from the computer to the serial device.
; Pin 4 - Data Terminal Ready (DTR) indicates to the modem that the computer is ready to
; Pin 5 - Ground.
; Pin 6 - Data Set Ready (DSR) is similar to DTR. It indicates that the Dataset is ON.
; Pin 7 - The RTS pin requests clearance to send data to a modem.
; Pin 8 - The serial device uses the Clear to Send (CTS) pin to acknowledge the RTS signal of the
computer. In most situations, RTS and CTS are constantly ON throughout the communication
; Pin 9 - An auto answer modem uses the Ring Indicator (RI) to signal receipt of a telephone
The DCD and RI pins are only available in connections to a modem. These two lines are used rarely because most modems transmit status information to a PC when a carrier signal is detected (when a connection is made to another modem) or when the modem receives a ring signal from the telephone line.
2.1.2 - Time Division Multiplexing (TDM)
The diagram depicts the concept of Time Division Multiplexing (TDM).
In the example shown, a multiplexer (MUX) at the transmission end accepts three separate signals, a video camera, a voice switch, and a router. The MUX breaks each signal into segments and puts each segment into a single channel by inserting each segment into a timeslot. At the receiving end, another MUX separates the single serial transmission stream into the three original ones and sends the output to the appropriate device. There are eight bits per timeslot (TS). Timeslots TS 0 through TS 31 are shown.
- TDM shares available transmission time on a medium by assigning timeslots to users. - The MUX accepts input from attached devices in a round-robin fashion and transmits the data in a
- T1/E1 and ISDN telephone lines are common examples of synchronous TDM.
Time Division Multiplexing
Bell Laboratories invented time-division multiplexing (TDM) to maximize the amount of
voice traffic carried over a medium. Before multiplexing, each telephone call required its own physical link. This was an expensive and unscalable solution. TDM divides the bandwidth of a single link into separate channels or time slots. TDM transmits two or more channels over the same link by allocating a different time interval (time slot) for the transmission of each channel. In effect, the channels take turns using the link.
TDM is a Physical layer concept. It has no regard for the nature of the information that is being multiplexed onto the output channel. TDM is independent of the Layer 2 protocol that has been used by the input channels.
TDM can be explained by an analogy to highway traffic. To transport traffic from four roads to another city, you can send all the traffic on one lane if the feeding roads are equally serviced and the traffic is synchronized. So, if each of the four roads puts a car onto the main highway every four seconds, the highway gets a car at the rate of one each second. As long as the speed of all the cars is synchronized, there is no collision. At the destination, the reverse happens and the cars are taken off the highway and fed to the local roads by the same synchronous mechanism.
This is the principle used in synchronous TDM when sending data over a link. TDM increases the capacity of the transmission link by slicing time into smaller intervals so that the link
carries the bits from multiple input sources, effectively increasing the number of bits transmitted per second. With TDM, the transmitter and the receiver both know exactly which signal is being sent.
In our example, a multiplexer (MUX) at the transmitter accepts three separate signals. The MUX breaks each signal into segments. The MUX puts each segment into a single channel by inserting each segment into a timeslot.
A MUX at the receiving end reassembles the TDM stream into the three separate data streams
based only on the timing of the arrival of each bit. A technique called bit interleaving keeps track of the number and sequence of the bits from each specific transmission so that they can be quickly and efficiently reassembled into their original form upon receipt. Byte interleaving performs the same functions, but because there are eight bits in each byte, the process needs a bigger or longer time slot.
2.1.2 - Time Division Multiplexing (TDM)
The diagram depicts the concept of Statistical Time Division Multiplexing (STDM). The diagram is the same as diagram 2.1.1, except that the MUX is labeled STDM MUX.
Statistical Time Division Multiplexing
In another analogy, compare TDM to a train with 32 railroad cars. Each car is owned by a different freight company, and every day the train leaves with the 32 cars attached. If one of the companies has cargo to send, the car is loaded. If the company has nothing to send, the car remains empty but stays on the train. Shipping empty containers is not very efficient. TDM shares this inefficiency when traffic is intermittent, because the time slot is still allocated even when the channel has no data to transmit.
Statistical time-division multiplexing (STDM) was developed to overcome this inefficiency.
STDM uses a variable time slot length allowing channels to compete for any free slot space. It employs a buffer memory that temporarily stores the data during periods of peak traffic. STDM does not waste high-speed line time with inactive channels using this scheme. STDM requires each transmission to carry identification information (a channel identifier).
2.1.2 - Time Division Multiplexing (TDM)
The diagram depicts TDM examples of ISDN and synchronous optical networking (SONET). Also shown are DS0 (digital signal level zero) units and the T-Carrier hierarchy.
The diagram shows ISDN Basic Rate Interface (BR I) with three channels consisting of two 64 kilobits per second B-channels (B1 and B2), and a 16 kilobits per second D-channel. The TDM has nine timeslots, which are bit-interleaved and repeated in the sequence B1, B2, B1, B2, B1, B2, B1, B2, and D.
Two users are shown connected to an ISDN network termination type one (NT1) device and sharing the ISDN circuit of 64 kilobits per second B-channels. The control channel is the D channel.
The diagram shows an example of statistical TDM with SONET /SDH. In the diagram, optical traffic arrives at the SONET multiplexer from four places at 2.5 Gigabits per second and goes out as a single stream at 4 times 2.5 Gigabits per second, or 10 Gigabits per second.
The diagram presents a table of DS bit types with the data rate and number of equivalent DS0 voice slots.
Signal Bit: DS0.
Rate: 64 kilobits per second.
Voice Slots: 1 DS0.
Signal Bit: DS1.
Rate: 1.544 Megabits per second.
Voice Slots: 24 DS0's.
Signal Bit: DS2.
Rate: 6.312 Megabits per second.
Voice Slots: 96 DS0's.
Signal Bit: DS3.
Rate: 44.736 Megabits per second.
Voice Slots: 672 DS0's or 28 DS1's.
The diagram presents the relationship between T-Carrier hierarchy units of transmission. T-carrier refers to the bundling of DS0's. In the diagram:
T1 = 24 DS0's (1.544 Megabits per second).
T1 C = 48 DS0's or two T1's (3.152 Megabits per second).
T2 = Two T1 C's (6.312 Megabits per second).
T3 = Seven T2's (45 Megabits per second).
T4 = Six T3's (274 Megabits per second).
TDM Examples - ISDN and SONET
An example of a technology that uses synchronous TDM is ISDN. ISDN basic rate (BRI) has three channels consisting of two 64 kb/s B-channels (B1 and B2), and a 16 kb/s D-channel. The TDM has nine timeslots, which are repeated in the sequence shown in the figure.
On a larger scale, the telecommunications industry uses the SONET or SDH standard for
optical transport of TDM data. SONET, used in North America, and SDH, used elsewhere, are two closely related standards that specify interface parameters, rates, framing formats, multiplexing methods, and management for synchronous TDM over fiber.
Click the SONET button in the figure.
The figure displays an example of statistical TDM. SONET/SDH takes n bit streams, multiplexes them, and optically modulates the signal, sending it out using a light emitting device over fiber with a bit rate equal to (incoming bit rate) x n. Thus traffic arriving at the SONET multiplexer from four places at 2.5 Gb/s goes out as a single stream at 4 x 2.5 Gb/s, or 10 Gb/s. This principle is illustrated in the figure, which shows an increase in the bit rate by a factor of four in time slot T.
Click the DS0 button in the figure.
The original unit used in multiplexing telephone calls is 64 kb/s, which represents one phone call. It is referred to as a DS-0 or DS0 (digital signal level zero). In North America, 24 DS0
units are multiplexed using TDM into a higher bit-rate signal with an aggregate speed of 1.544 Mb/s for transmission over T1 lines. Outside North America, 32 DS0 units are multiplexed for E1 transmission at 2.048 Mb/s.
The signal level hierarchy for multiplexing telephone calls is shown in the table. As an aside, while it is common to refer to a 1.544 Mb/s transmission as a T1, it is more correct to refer to it as DS1.
Click the T-Carrier Hierarchy button in the figure.